JP-2012-177303A shows a fuel injection controller which controls a fuel injector. The fuel injector has a coil. When the coil is energized, the coil generates an electromagnetic force which lifts up a valve body to inject a fuel. The fuel injection controller controls an energization start time of the coil and an energization period, whereby a fuel injection start time and a fuel injection quantity are controlled.
In the fuel injection controller, as shown in FIG. 7B, after an energization of the coil is started, a voltage-application is continued until the coil current reaches a target peak value Ipeak. The target peak value Ipeak is required to lift up the valve body to the maximum lift position.
The electric current required to hold the valve body at the maximum lift position is less that the target peak value Ipeak. Because, when the electromagnetic force is increased, a magnetic field change is large and a inductance is also large. Meanwhile, when the electromagnetic attracting force is kept at a constant value, the inductance is small.
In the above conventional controller, when the coil current reaches the target peak value, the coil current is decreased and is kept at a hold value (hold which is smaller than the target peak value Ipeak.
When a temperature of a coil is increased, an electric resistance of the coil is also increased. As shown by dashed lines in FIGS. 7A and 7B, a time period t10-t20 in which the coil current reaches the target peak value Ipeak becomes longer. As a result, since an increasing ratio ΔF of attracting force becomes smaller (dashed line in FIG. 7C), a valve opening start time “ta” is delayed and a valve opening period Tact becomes shorter.
According to a temperature characteristic of the coil current, an increasing ratio ΔI of the electric current is varied. As the result, the increasing ratio ΔF of attracting force is varied, so that the valve opening start timing “ta” and the valve opening period Tact are varied. That is, since the valve opening start timing “ta” and the valve opening period Tact receive an influence of the temperature characteristic, a fuel injection accuracy relative to the energization start time t10 and the energization period Ti is deteriorated.
Especially, in a case that a multi-stage injection is conducted in one combustion cycle, it is required that small amount fuel is injected with high accuracy. In such a small injection, a deviation of the injection start time “ta” becomes large, so that the injection accuracy due to the temperature characteristic is further deteriorated.